It's one thing to watch an eye-popping scene in a CG-laden blockbuster and go, "How the heck did they do that?" It's quite another to have the filmmakers behind those scenes pull back the cinematic curtain and explain what it took to put the footage up onscreen.

Now we turn our attention to "Pirates of the Caribbean: On Stranger Tides" and visual-effects supervisor Charlie Gibson to find out the secrets behind some of the new film's most stunning scenes, from Johnny Depp's perilous dive from a flaming lighthouse to an action sequence in which beautiful but dangerous mermaids go on the attack.

The Fiery Lighthouse

Depp is well into his late 40s, but he still took on a range of stunt work in "Stranger Tides." For some scenes, though, he deferred to stunt double Christopher Leps. But other scenes were so perilous, the only safe choice was computer technology. In what Gibson called one of the film's most challenging visual effects, filmmakers had to capture Depp's Captain Jack Sparrow leaping from a flaming lighthouse and into the water below. To reach that point, they integrated the two men's work with the capabilities of CGI.

"We had these guys come in with a full-body scanner and we grabbed Johnny after dinner and put him on a turntable and scanned him in so his likeness is in the computer," Gibson explained. "Then in the computer, we match his clothing and his hair and all the details from the scene, blending his actions and matching what he does before and after the scene. We'll have different pieces that we've shot all over the world. Some at Universal in [Los Angeles], some shot in a water tank at Pinewood Studios in London, and then the animated performance."

It's a bit like a virtual patchwork blanket — a labor-intensive process with so many different elements that, in the end, stitches together to create a unified, impressive whole. A veteran in dealing with this sort of thing, Depp was happy to go along for the ride. "He's good with it. We try to be quick with the scan," Gibson said. "But he's a good sport."

One of the film's most visually impressive scenes comes when Sparrow and his pirate pals find their way into a cove filled with mermaids. Lovely to look at, with their half-fish, half-human forms, the mermaids will attack when threatened. Creating them was no easy task.

"We didn't want to bother with any kind of makeup or animatronic approach," Gibson explained. "And we had this rule where the waterline defined where the mermaid stopped and started. So we knew we'd be showing a hybrid, and we had to come up with a very flexible solution."

"On the other movies, we were able to use these [motion-capture] tracking pajamas for certain characters," he added. "But when you have a naked mermaid, there are not a lot of tracking pajamas you can use. For the models playing the mermaids, we used these transfer tattoos where we put tracking marks on their skin and [Industrial Light & Magic] developed these skimpy bathing suits. We basically glued the [CGI] mermaid onto their bodies. When the mermaids were being played by the stunt performers or synchronized swimmers for acrobatic stuff, we used thicker wetsuits."

As you might remember from various trailers, "Pirates" features a ship that essentially comes alive, as the villain Blackbeard (Ian McShane) controls the sails and riggings. He's able to garner control of the vessel simply by moving his sword, as ropes fly about and toss sailors to and fro. The scene gets even more impressive when you realize it was done almost entirely through practical stunt work.

"That scene was 80 percent stunt performing," Gibson told us. "Those guys were really flying through the air. We threw some flying ropes in there and erased their stunt rigs. It's exciting to watch even before we go in and sweeten it."

"When the boat breathes fire, that's a collaboration between us and the practical effects guys," he added. "We add in more fire, bring it closer to the performer. When the fire is really there, you see it reflected in the water and the people's faces and it looks 100 times more real."

The driving story of the film is the quest for the Fountain of Youth. When Jack and his cohorts first arrive at what they believe to be the fountain's location, they don't see much. But further investigation shows them it's been hiding there all along — the setting magically floods with water and the fountain is revealed. It's a dazzling bit of visual-effects work, with gravity playing all sorts of tricks and the water seeming to have an intelligence all its own. To get the scene right, Gibson and his team turned to Scanline VFX, an effects house nominated at this year's Oscars.

"I wanted to find a way to have the entrance to the fountain be in plain sight but somehow not be there. People have been looking for it for years but somehow haven't found it. Why did they find it and why now?" Gibson said. "We were lucky enough to hook up with these great water people, the same guys who did the huge tsunami for 'Hereafter.' What they came up with was amazing."